1. Field of the Invention
The present invention relates to an air-fuel ratio control for an internal combustion engine system which includes a catalytic converter for treating the exhaust gases and reducing emissions, and more specifically to an air-fuel control arrangement for such a system wherein secondary air is introduced into the exhaust gases at a location upstream of the catalytic converter.
2. Description of the Prior Art
JP-A-60-240840 discloses an arrangement wherein engine speed M and induction air quantity Q are used do determined the amount of air which is being inducted into the engine cylinder and to derive a basic injection fuel amount Tp (viz., T=K.multidot.Q/N). This basic injection pulse width Tp is then modified using a correction factor based on engine coolant temperature and the like type of parameter, and a feedback control correction factor which is based on the data fed back from an O.sub.2 sensor disposed in the exhaust system.
In order to improve emission control it is known to dispose a three-way catalytic converter in the exhaust system and control the air-fuel ratio (A/F) as close to the stoichiometric air-fuel ratio as possible in order to maximize the efficiency with which CO and HC are oxidized and NOx is reduced.
In order to maintain the A/F as close to the stoichiometric ratio as possible, it is known to utilize two O.sub.2 sensors in an arrangement wherein one is disposed upstream of the catalytic converter and the other downstream thereof. The output of the first upstream device is subjected to P.I. control and basically used for air-fuel ratio control while the downstream one is used to adjust a correction factor which is applied to the output of the upstream sensor and compensate for the change in output characteristics which occur with the passing of time. That is to say, O.sub.2 sensors which are disposed upstream of the catalytic converters are exposed to conditions which tends induce the degradation of the same.
In order to stabilize engine idling, it has been proposed to clamp the air-fuel ratio control in a manner which forms a rich air-fuel mixture and to supply secondary air into the exhaust gases in order to promote the oxidation of the CO and HC.
However, in the event that a dual O.sub.2 sensor system is used, even through the secondary air is introduced into the exhaust gases at a location between the upstream O.sub.2 sensor and catalytic converter, the downstream O.sub.2 sensor is exposed to exhaust gases which exhibit and A/F which includes the secondary air. As a result the output of the O.sub.2 sensor tends to effect erroneous adjustments to the correction factors used to determined the air-fuel ratio control, and when the mode of engine operation changes from idling to a higher load mode, the supply of secondary air is terminated, the arbitrary fuel enrichment stopped and the air-fuel ratio control again performed in accordance with the output of the O.sub.2 sensors; due to the modification of the correction factor(s) during the idling period, during the initial period following the end of the idling mode of operation, the air-fuel ratio is deteriorated often to the point of deteriorating the engine performance characteristics.